Haarlem, 1623. Gilda Beresteyn overhears a conspiracy involving her brother and Willem van Oldenbarnevelt, Lord of Stoutenburg, who seeks revenge on Maurice of Nassau for the execution of his father. To prevent Gilda warning the stadtholder, the conspirators hire an English soldier of fortune known as Diogenes to abduct and hold her until their plan has been carried out.
Diogenes begins as a mercenary concerned with payment, yet Gilda’s courage and the treachery surrounding the commission force him to choose a side. Frans Hals appears within the story as the painter who captures the adventurer’s insolent confidence in the portrait later called The Laughing Cavalier. Orczy uses the real 1623 conspiracy against Maurice of Nassau as the foundation for an invented Blakeney ancestor whose appetite for disguise, rescue and risk anticipates the Scarlet Pimpernel.